This invention relates to video tape recorder/reproducer systems and, in particular, to a measuring circuit for accurately determining the position of the magnetic tape.
In the operation of magnetic tape recorders, it is often required that the position of the magnetic tape, with respect to its beginning, be accurately known. Generally, this is accomplished by means of an optical or magnetic tachometer coupled to the capstan or to a roller in the tape path. The output pulses of the tachometer are processed in related circuitry to provide a tape-timer output based on the relationship which exists between the number of tachometer pulses and the image or picture frames recorded per unit measure of the magnetic tape. In this manner, the position of the tape may be expressed in terms of hours, minutes, seconds and, if desired, actual number of frames. Although widely used in commercial practice, such systems are not capable of achieving the desired or optimum accuracy because of the mechanical interface between the magnetic tape and the tachometer drive roller. The accuracy of the tape-timer readout is significantly affected, for example, by inaccurate and changing drive roller dimensions, as well as tape slippage between the magnetic tape and the drive roller. It has been proposed to utilize a prerecorded signal, identified as the control-track signal, for the tape-timer source of signals. The control-track signal is recorded on the magnetic tape simultaneously with the video signals at a rate of eight cycles per picture frame in the NTSC standards, or at a rate of ten cycles per frame for the PAL and SECAM standards. The control track is a permanent recording on the magnetic tape directly related to time and, as such, is not affected by the mechanical interface problems described above. However, not all recorded magnetic tapes contain this control track signal, and in many magnetic recordings, the control-track signal is not continuous; that is, it is intermittent due to splices, editing, and other related signal processing steps. In view of the inconsistent availability of the control-track signal, the advantages of a more accurate and repeatable display of the tape position made possible by counting cycles of the periodic control track signal have not been heretofore realized.